Pork tenderloins typically come two to a package or as one large tenderloin. Either will work. Personally, whenever I can find Niman Ranch pork I purchase it as they are known for excellent quality pork that has been humanely raised from birth.
If the cider reduction gets too thick or reduces faster than you intended, simply add a little more juice or even water. I personally like lots of sauce. If you do as well, increase the sauce ingredients by a couple of tablespoons each.